" That wall," explained the Shaggy Man, "is what is called an optical illusion. It is quite real while you have your eyes open, but if you are not looking at it the barrier doesn't exist at all. It's the same way with many other evils in life; they seem to exist, and yet it's all seeming and not true. "

― L. Frank Baum , Complete Works of L. Frank Baum "American Author of Children's Books"! 45 Complete Works (American Fairy Tales, Aunt Jane's Nieces Series, Wizard of Oz Series, Mother Goose in Prose) (Annotated)